1. Field of the Invention
The present invention relates generally to an objective lens driving apparatus for use with an optical pickup device of an optical disc recording and/or reproducing apparatus. More particularly, this invention relates to a supporting structure for supporting a movable portion including an objective lens so that the movable portion can be freely moved.
2. Description of the Prior Art
Generally, an optical disc apparatus utilizes an optical pickup device to write and/or read a signal in and/or from an optical disc. The optical pickup device includes a light source such as a semiconductor laser element or the like. A laser beam emitted from the light source is focused on a signal recording surface of an optical disc and a laser beam reflected on the signal recording surface is detected by an optical detecting element. An information signal is thereby written on and/or read from the optical disc.
The optical pickup device includes an objective lens driving apparatus to move the objective lens in two directions, i.e., the direction (focusing direction) parallel to the optical axis of the objective lens and the direction (tracking direction) perpendicular to the optical axis of the objective lens by an electromagnetic driving force. By virtue of the objective lens driving apparatus, the laser beam can accurately scan recorded tracks formed on the signal recording surface of the optical disc.
This type of objective lens driving apparatus according to the prior art employs a leaf spring as a supporting member for moving the objective lens 50 that the objective lens can be moved in the two directions (see Japanese Patent Application No. 63-334904 previously proposed by the assignee of the present invention).
FIG. 1 shows an arrangement of this type of objective lens driving apparatus.
It will be seen in FIG. 1 that an objective lens 1 is supported by a bobbin 2. A focusing coil 3a and tracking coils 3b are mounted on the bobbin 2 as electromagnetic driving means for driving the objective lens 1. The bobbin 2 is supported by a pair of supporting members 4 and 5 provided at the left and right sides of the bobbin 2 so that it can be moved in the first direction (focusing direction, i.e., vertical direction) parallel to the optical axis of the objective lens 1 and in the second direction (tracking direction, i.e., horizontal direction) perpendicular to the optical axis of the objective lens 1. The supporting members 4 and 5 are formed of leaf springs such as a stainless steel leaf spring having a predetermined rigidity. The supporting members 4 and 5 are the same in shape and are located symmetrically to each other.
More specifically, each of the leaf springs forming the supporting members 4 and 5 is integrally composed of front and rear supporting plate portions 6 and 7 and upper and lower arm portions 8 and 9 coupling the two supporting plate portions 6 and 7. The arm portions 8 and 9 are bent horizontally relative to the vertical supporting plate portions 6 and 7. Slits 6b and 7b are formed through the vertical supporting plate portions 6 and 7, respectively, so that displacement portions 6a and 7a are formed on the supporting plate portions 6 and 7 thereby to enable the objective lens 1 to be moved in the direction perpendicular to the optical axis, i.e., the tracking direction.
There is shown a tab 10 which is bent in order to maintain a predetermined rigidity of the supporting members 4 and 5 in the vertical direction. Rectangular apertures 11 are formed through the supporting plate portions at one end side, i.e., front side supporting plate portions 6. When protrusions 12 projected from the side portion of the bobbin 2 are in engagement with the rectangular apertures 11 and the heads of the protrusions 12 are caulked, the bobbin 2 is secured to the front ends of the supporting members 4 and 5.
Apertures 13 are formed through the other end side, i.e., rear supporting plate portions 7, and the supporting members 4 and 5 are attached to a fixed base portion 15 by engaging fixing members 14 in the apertures 13. More precisely, apertures 16 formed through two side portions of the base portion 15 are made corresponding with the apertures 13 of the supporting members 4 and 5 and protrusions 14a of the fixing members 14 penetrate into the apertures 13 and 16 to be secured thereto.
In this objective lens driving apparatus, a magnet and a yoke are provided at the stationary side in an opposing relation to the focusing coil 3a and the tracking coils 3b and a magnetic circuit is constructed between the magnet and the yoke. The magnet and yoke are not shown in FIG. 1 because they are not so important in order to explain the present invention.
In the objective lens driving apparatus thus constructed, the bobbin 2 is supported by the left and right supporting members 4 and 5 so that the bobbin 2 can be moved in the two directions. When the tracking coils 3b are supplied with a predetermined tracking current, the displacement portions 6a and 7a of the two supporting members 4 and 5 are flexed in the horizontal direction in parallel to each other, whereby the bobbin 2 is positionally moved in the second direction perpendicular to the optical axis of the objective lens 1 and the tracking is performed. When on the other hand the focusing coil 3a is supplied with a predetermined focusing current, the arm portions 8, 8 and 9, 9 of the two supporting members 4 and 5 are flexed in the vertical direction in parallel to each other, whereby the bobbin 2 is positionally moved in the first direction parallel to the optical axis of the objective lens 1 and the focusing of the laser beam is performed.
In the above-described prior-art objective lens driving apparatus, the supporting members for supporting the objective lens are independently provided at the left and right sides of the bobbin so that characteristics (spring constant, resonance frequency, etc.) of the leaf springs forming the two supporting members fluctuate. As a result, the objective lens is not driven regularly, causing the optical axis of the objective lens 1 to be inclined or displaced from the correct state. This phenomenon will be hereinafter referred to as a so-called optical axis inclination or irregularity for simplicity. To avoid this defect, the characteristics of the two leaf springs must be made the same, which requires a strict administration of assembly parts. Consequently, the objective lens driving apparatus become quite expensive. Further, in the assembly process, the two supporting members must be secured with high positional accuracy, which as a result requires the increase of the assembly process. Therefore, the objective lens driving apparatus according to the prior art becomes further expensive. As described above, the prior-art objective lens driving apparatus has various problems to be solved.